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Chemical reactions: blow hot, blow cold

Life seems to thrive on the fact that warm and cold are, well..., two different temperature ranges! It is November now, and the northern hemisphere heads into winter just as the southern hemisphere starts heating up to start up their summers.

This magical arrangement is caused by the Earth's tilt, the same feature that creates the weather (winds, rain, storms, clouds, precipitation), and the seasons, and many things that are interesting about the Earth's surface.

As if to mirror this property of Nature, every known chemical reaction is either endothermic (requiring, or drawing in heat) or exothermic (giving off heat). An easy way to remember which is which, is to remember that "endo" is Latin for within and sounds like "indo" - heat is pulled in. Exo is Latin for outside, so it describes reactions that give off heat, sending heat outside. Simply put, an exothermic reaction gives off heat and feels warm.

When two or more chemicals react, it is really their molecules that come up against each other and react. Some molecules have high valencies (available, unpaired electrons) and so react easily at room temperature, usually releasing heat energy when the chemical bonds are broken. Other molecules need to be heated to reach a level of energy at which old chemical bonds can be broken to form new ones. The former are exothermic, and the latter are endothermic. Chemical reactions that occur in nature tend not to be thought of as "chemical reactions" because there is no laboratory in sight, but they are valid chemical reactions all the same - the formation of ammonia in a lightning storm, the rusting of iron, the fermentation of barley and hops to form beer, and so on.

Rusting iron gives off heat, whereas the fermentation in baking bread requires heat. Making ice cubes gives off heat (surprising? But unless a body of water lost more heat than it absorbed, it could not get colder, and turn to ice) whereas melting ice absorbs heat and so is endothermic.

Can you think of other commonly seen reactions, and classify them as endothermic or exothermic?